Galvanized steel sheets are used in a wide variety of fields, typically in automotive body applications. Galvanized steel sheets in automotive body applications are subjected to press forming and painting before use.
However, one drawback of galvanized steel sheets is that they have lower press formability than cold-rolled steel sheets. This is because galvanized steel sheets have higher sliding resistance on press dies than cold-rolled steel sheets. More specifically, high sliding resistance between a press die and a bead often hampers a galvanized steel sheet from entering the press die, thus causing the galvanized steel sheet to fracture.
A method of applying a high-viscosity lubricating oil is widely used as a method for improving press formability of galvanized steel sheet during use. In this method, however, running out of oil in press forming results in unstable press performance. Thus, galvanized steel sheets are strongly required to have improved press formability by themselves.
In recent years, attempts have been made to simplify manufacturing processes and reduce environmentally-hazardous substances in manufacturing processes. In particular, in an alkaline degreasing process, which is a pretreatment process before a painting process, progress is being made in decreasing the process line length and the temperature of the work environment. Thus, there is a demand for galvanized steel sheets having good degreasing property without adversely affecting the painting process even under such severe conditions.
Thus, there is a demand for a galvanized steel sheet for use in automobiles that has good press formability and good degreasing property even under severer alkaline degreasing treatment conditions than before.
A technique for improving press formability may be a technique of forming a lubricating film on the surface of galvanized steel sheet or a technique of forming an oxide layer on the surface of galvanized steel sheet.
Patent Literature 1 discloses a technique for improving press formability and chemical conversion treatability by producing Ni oxides on the surface of galvanized steel sheet by electrolysis treatment, dip treatment, painting oxidation treatment, or heat treatment.
Patent Literatures 2 and 3 disclose a technique for improving sliding characteristics by bringing a galvannealed steel sheet into contact with an acidic solution to form an oxide layer composed mainly of Zn oxides on the surface of galvannealed steel sheet, thereby suppressing adhesion between the galvannealed layer and a press die.
A technique for improving degreasing property may be a technique of washing a galvannealed steel sheet with an alkaline solution or a solution containing phosphorus (P).
Patent Literature 4 describes a technique for improving degreasing property by washing the surface of galvannealed steel sheet with an alkaline solution.
Patent Literature 5 describes a technique for improving degreasing property by washing the surface of galvannealed steel sheet with a solution containing P.